A read-only memory is a matrix network forming a grid having rows forming selected words and columns forming binary units or bits of the words. The correspondence of the bits to a given word is determined by memory elements which link the row for the word to the bit columns. The memory elements are arranged at intersections of the grid forming the memory in such a way that the arrangement is fixed.
When read-only memories are being manufactured, all the intersections are sometimes provided with destructible linking elements so that it is subsequently possible for the user to create a suitable pattern of links in the matrix network of the memory by destroying certain of the linking elements of the network. A programming operation is thereby performed and the memory is termed programmable.
Destructible memory elements may be divided into two categories: those which initially form a conductive link between the rows and the columns and can be destroyed by a current overload, these being for instance elements made of a fusible material which create an open circuit when they are destroyed; and those which initially create a barrier to any linking, such as diodes which are intended to be reverse biased and which can be destroyed by causing them to break under a current or voltage overload, after which they form short circuits when the memories are in normal use. Consequently, broadly speaking, programming consists in applying an electrical overload to an element to be destroyed by selecting the word row and bit column to which the element is connected. The conductors in the matrix network therefore must carry this overload without substantial loss so that the complete destruction of the selected memory element is assured to prevent the overload from being too small. Too small an overload may occur with certain memories which are integrated into semiconductor substrates, in which the rows, for example, are formed by doping bands within the substrate. These bands are mutually parallel straight lines of relatively greater resistance than metal wires which intersect with them and are applied to the substrate on a dielectric layer to form the columns of the matrix network of the memory. These columns are connected to the bands by the destructible linking elements.
To avoid losses in the resistive rows when programming integrated read-only memories, one solution is to provide the read-only memory with shunt paths which are good conductors and are intended to divert the programming currents flowing in the resistive bands formed in the semiconductor substrate. The shunt paths are connected to the bands by semiconductor structures whose conductive state is controlled by applying a control potential to the band to which the memory element to be destroyed corresponds.
In this way, the programming current of the designated wire passes through the controlled conduction semiconductor structure and returns along the appropriate shunt path, which may be a metal wire similar to the metal wires forming the columns of the read-only memory. The aforementioned structure is formed by four layers of alternating conductivity types, these layers being arranged within the semiconductor bands forming the rows of the memory.
The semiconductor material of these bands may be used as the material of one layer of the structure, namely the layer which acts as the control electrode or gate which actuates the structure into a conducting state. When the destructible element is a diode formed by two semiconductor layers of opposite conductivity, one of which is connected to a bit wire, the other layer may take the place of one of the four layers of the controlled conduction structure.
In such a structure, if the memory contains P columns, there are at least P/2 shunt paths for the programming currents. The presence of a large number of shunt paths complicates the structure of the memory and, when the memory is of a given size, prevents the accommodation of a large number of rows and columns. An example of such a structure is described in British Pat. No. 1440167.